1. Field of the Invention
This invention relates to an optical recording medium capable of optically recording and reproducing information thereon and to a reproducing apparatus. Description of the Prior Art
Optical disks have heretofore been put into practical use as large capacity recording media.
The optical disks are generally divided into two types, i.e., the type which reproduces recorded information from a variation in the reflectance of a recording track and the type which reproduces recorded information from the rotation of the plane of polarization of a light beam incident on a recording track.
In the optical disk which utilizes a variation in reflectance, a physical pit corresponding to the recorded information is formed on the recording track or the crystalline structure is changed, and reproduction of the recorded signal is effected from a variation in the intensity of reflected light resulting from the presence of the pit or the difference in crystalline structure.
On the other hand, in the magnetooptic disk which utilizes the rotation of the plane of polarization of light, the direction of vertical magnetization of a recording track is varied in conformity with recorded information, and the rotation of the plane of polarization of the reflected light from the disk corresponding to the direction of vertical magnetization by the Kerr effect or the rotation of the plane of polarization of the light transmitted through the disk corresponding to the direction of vertical magnetization by the Faraday effect is detected, whereby signal reproduction is accomplished.
However, in such optical disks according to the prior art, if the spot diameter of a light beam used for signal reproduction is determined, the pitch of tracks cannot be made less than a predetermined value due to the tolerance limit of the crosstalk between adjacent tracks in the disk. Therefore, the recording density determined by the pitch of tracks cannot be exceeded, and this has led to a limited recording capacity.
Among the disks which utilize the rotation of the plane of polarization of light, there is one in which, as disclosed in U.S. Pat. No. 4,467,383, there are provided recording tracks whose direction of vertical magnetization is varied in conformity with recorded information and tracks for tracking arranged alternately with the recording tracks for the tracking of the disk. The surface of these tracks for tracking is flush with the surface of the recording tracks and is treated so that it has a predetermined reflectance different from that of the surface of the recording tracks.
Now, in the disk described in the aforementioned U.S. Pat. No. 4,467,383, the formation of the tracks for tracking is executed by applying a light beam or the like of high output to an amorphous layer provided on the disk (a layer which rotates the plane of polarization of reflected or transmitted light relative to the plane of polarization of linearly polarized incident light by the direction of vertical magnetization) to thereby shift the amorphous state to a crystalline state. The formation of the tracks for tracking requires a highly accurate apparatus and much time and is costly for mass production.